Voltage equalization device, method, program, and power storage system

ABSTRACT

A voltage equalization device of a power storage device provided with battery packs in which a plurality of secondary cells are connected, power converters provided in association with the battery packs, and controllers that control the power converters, and in which the battery packs are connected in parallel via the individual power converters includes a decision portion and a voltage-adjusting portion. The decision portion obtains battery-pack information regarding the states of charge/discharge of the individual battery packs and decides, for each battery pack, whether or not to perform voltage adjustment on the basis of the battery-pack information. When the decision portion decides that the voltage adjustment is to be performed, the voltage-adjusting portion generates offset instructions for adjusting the states of charge/discharge and outputs the offset instructions to the controllers of the power converters associated with the battery packs.

TECHNICAL FIELD

The present invention relates to a voltage equalization device, method,and program, as well as to a power storage system.

BACKGROUND ART

In the related art, when increasing the capacity of lead-acid batteries,single batteries or battery packs formed by connecting single batteriesin series (a battery pack may have not only portions connected in seriesbut also portions connected in parallel) are connected in parallel asshown in FIG. 6. For example, vehicle lead-acid batteries or the likegenerally have a large resistance of about 10 mΩ and, even if there areslight voltage differences caused by impedance or contact resistance dueto manufacturing variations, the voltage differences are absorbed bythis resistance, thus keeping the current bias in the individualbatteries small.

In recent years, however, more efficient secondary cells, such aslithium-ion batteries or the like, having lower internal impedance thanconventional ones are becoming available, and when batteries aredirectly connected in parallel in this case, slight differences inimpedance or contact resistance due to variability in manufacturing maycause excessively large current to flow at one side of the batterypacks. Thus, in recent years, power converters are connected to batterypacks, as shown in FIG. 7, and an overcurrent is prevented from flowingin the battery packs by controlling the current that flows in theindividual battery packs with the power converters.

CITATION LIST Patent Literature

-   {PTL 1} Japanese Unexamined Patent Application, Publication No.    2003-111288.

SUMMARY OF INVENTION Technical Problem

Even if the power converters are provided in association with theindividual battery packs, as shown in FIG. 7, however, variability inbattery conditions inevitably arises among battery packs, and there is aproblem in that it is difficult to use the individual battery packs inpreferable conditions by keeping the state of charge, etc. of therespective battery packs within a predetermined range.

The present invention has been conceived in order to solve theabove-described problems, and an object thereof is to provide a voltageequalization device, method, and program, as well as a power storagesystem, that are capable of equalizing the state of charge of batterypacks connected in parallel.

Solution to Problem

In order to solve the above-described problems, the present inventionemploys the following solutions.

The present invention provides a voltage equalization device for a powerstorage device provided with battery packs in which a plurality ofsecondary cells are connected, power converters provided in associationwith the battery packs, and controlling means for controlling the powerconverters, and in which the battery packs are connected in parallel viathe individual power converters, the voltage equalization deviceincluding deciding means for obtaining battery-pack informationregarding states of charge/discharge of the individual battery packs andfor deciding, for each battery pack, whether or not to perform voltageadjustment on the basis of the battery-pack information; andvoltage-adjusting means for generating, when the deciding means decidesthat the voltage adjustment is to be performed, offset instructions foradjusting the states of charge/discharge and for outputting the offsetinstructions to the controllers of the power converters associated withthe battery packs.

With such a configuration, in the power storage device in which theplurality of the battery packs are connected in parallel via the powerconverters, the deciding means decides, for each battery pack, whetheror not to perform the voltage adjustment on the basis of thebattery-pack information about the individual battery packs; when thedeciding means decides that the voltage adjustment is to be performed,the voltage-adjusting means outputs the offset instructions for thevoltage adjustment; and the power converters carry out charge/dischargecontrol of the battery packs on the basis of the offset instructions.Accordingly, voltage of each battery pack can be controlled and voltagedifferences in the individual battery packs can be eliminated.

With the voltage equalization device described above, the deciding meansmay decide whether or not to perform voltage adjustment by comparing theindividual items of battery-pack information with a predeterminedthreshold.

In this way, by comparing the individual items of battery-packinformation with the predetermined threshold, whether or not to performthe voltage adjustment can be decided with a simple method.

The above-described voltage equalization device may include a pluralityof bypass impedance elements that are connected in parallel with thebattery packs; switching elements that are connected in series with thebypass elements; and switching-element controlling means for controllingthe switching elements on the basis of the battery-pack informationregarding the states of charge/discharge of the individual batterypacks.

In this way, the bypass impedance elements are connected in parallelwith the individual battery packs and the switching elements areconnected in series with the bypass impedance elements; therefore, whenthe switching elements are turned on, voltages of the battery packsconnected in parallel with the bypass impedance elements can be lowered.Accordingly, when the bypass impedance elements are turned on, itbecomes possible to quickly lower the voltages as compared with loweringthe voltages based only on the offset instructions output from thevoltage-adjusting means. The bypass impedance elements described aboveare, for example, resistance components.

The above-described voltage equalization device may include a pluralityof bypass impedance elements that are provided for individual unit cellsconstituting the battery packs or for each cell group formed by dividingthe battery packs into a plurality of groups; switching elements thatare connected in series with the bypass impedance elements; andswitching-element controlling means for controlling the switchingelements on the basis of the battery-pack information regarding thestates of charge/discharge of the individual battery packs.

In this way, the bypass impedance elements are connected in parallelwith each of the individual cells that constitute the individual batterypacks or with each of cell groups formed by dividing the battery packsinto the plurality of groups, and the switching elements are connectedin series with the bypass impedance elements; therefore, when theswitching elements are turned on, voltages of the battery packsconnected in parallel with the bypass impedance elements can be lowered.Accordingly, by controlling the bypass impedance elements provided foreach of the individual cells that constitute the battery packs or foreach of cell groups formed by dividing the battery packs into theplurality of groups, it becomes possible to quickly lower the voltagefor each cell or each cell group.

The present invention provides a power storage system provided with apower storage device that includes battery packs in which a plurality ofsecondary cells are connected, power converters individually providedfor the battery packs, and controlling means for controlling the powerconverters, and in which the battery packs are connected in parallel viathe individual power converters; and a voltage equalization deviceaccording to one of claims 1 to 4.

The present invention provides a voltage equalization device for a powerstorage device provided with battery packs in which a plurality ofsecondary cells are connected and power converters provided inassociation with the battery pack, and in which the battery packs areconnected in parallel via the individual power converters, wherein oneof the battery packs is connected in parallel with other battery packsother than the one battery pack via an impedance element.

With such a configuration, in the power storage device in which theplurality of the battery packs are individually connected in parallelvia the power converters, one of the battery packs and the other batterypacks other than the one battery pack are connected in parallel via theimpedance element. Therefore, when there are voltage differences betweenthe connected battery packs, current flows through the impedanceelements connected so as to eliminate the voltage differences. As aresult, voltage for each battery pack is controlled and it becomespossible to eliminate the voltage differences between the battery packs.

In the above-described voltage equalization device, adjacent batterypacks may be connected in parallel with each other via impedanceelements, and two battery packs located at both ends are connected inparallel via an impedance element.

With the above-described configuration, the adjacent battery packs areconnected in parallel with each other via the impedance elements and thetwo battery packs located at both ends are connected via the impedanceelement; therefore, the battery packs can be connected with each otherin a ring-like manner via the impedance elements. Accordingly, ascompared with connecting one of the battery packs with the other batterypacks via the impedance element, the number of current routecommunicating from the high-voltage battery packs to the low-voltagebattery packs can be increased and the efficiency of the voltageequalization can be enhanced.

In the above-described voltage equalization device, the battery packsamong the plurality of the battery packs that are not adjacent to eachother may be connected in parallel with each other via impedanceelements.

Because the battery packs among the plurality of the battery packs thatare not adjacent to each other are connected with each other via theimpedance elements in this way, the number of current routescommunicating from the high-voltage battery packs to the low-voltagebattery packs can be further increased and the efficiency of the voltageequalization can be further enhanced.

The above-described voltage equalization device may include switchingelements that are connected in series with the impedance elements; andswitching-element controlling means for obtaining respectivebattery-pack information for the battery packs and for controlling theswitching elements on the basis of the battery-pack information.

With such a configuration, by turning on the switching elements on thebasis of the individual items of the battery-pack information obtainedfrom the individual battery packs, it becomes possible to performcharging/discharging of the battery packs via the impedance elements.Accordingly, it becomes possible to selectively performcharging/discharging of the battery packs via the impedance elements.

In the above-described voltage equalization device, theswitching-element controlling means may perform on/off control of theswitching elements by comparing the battery-pack information with apredetermined threshold.

In this way, by deciding whether or not to perform the on/off control ofthe switching elements by comparing the individual items of battery-packinformation with the predetermined threshold, it becomes possible toselectively perform the voltage adjustment only for the battery packswith large voltage differences.

The present invention provides a voltage equalization method for a powerstorage device provided with battery packs in which a plurality ofsecondary cells are connected, power converters provided in associationwith the battery packs, and controlling means for controlling the powerconverters, and in which the battery packs are connected in parallel viathe individual power converters, the voltage equalization methodincluding a deciding step of obtaining battery-pack informationregarding state of charge/discharge of the individual battery packs anddeciding, for each battery pack, whether or not to perform voltageadjustment on the basis of the battery-pack information; and anoutputting step of generating, when it is decided in the deciding stepthat the voltage adjustment is to be performed, offset instructions foradjusting the states of charge/discharge and outputting the offsetinstructions to the controllers of the power converters associated withthe battery packs.

The present invention provides voltage equalization program to beapplied to a voltage equalization device of a power storage deviceprovided with battery packs in which a plurality of secondary cells areconnected, power converters provided in association with the batterypacks, and controlling means for controlling the power converters, andin which the battery packs are connected in parallel via the individualpower converters, the voltage equalization program causing a computer toexecute decision processing for obtaining battery-pack informationregarding state of charge/discharge of the individual battery packs anddeciding, for each battery pack, whether or not to perform voltageadjustment on the basis of the battery-pack information; and outputtingprocessing of generating, when it is decided in the decision processingthat the voltage adjustment is to be performed, offset instructions foradjusting the states of charge/discharge and outputting the offsetinstructions to the controllers of the power converters associated withthe battery packs.

The present invention provides a voltage equalization method for a powerstorage device with battery packs in which a plurality of secondarycells are connected, and power converters provided in association withthe battery packs, and in which the battery packs are connected inparallel via the individual power converters, wherein one of the batterypacks and other battery packs other than the one battery pack areconnected in parallel via an impedance element.

Advantageous Effects of Invention

With the present invention, an advantage is afforded in that the stateof charge can be equalized among battery packs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing, in outline, a power storage systemaccording to a first embodiment of the present invention.

FIG. 2 is a block diagram showing the internal configuration of avoltage equalization device.

FIG. 3 is an operation flow chart of the power storage system accordingto the first embodiment of the present invention.

FIG. 4 is a block diagram showing, in outline, a power storage systemaccording to a second embodiment of the present invention.

FIG. 5 is a block diagram showing, in outline, a power storage systemaccording to a third embodiment of the present invention.

FIG. 6 is a diagram showing, in outline, conventional battery packs.

FIG. 7 is a diagram showing, in outline, the conventional battery packsconnected by using power converters.

DESCRIPTION OF EMBODIMENTS

Embodiments of a voltage equalization device according to the presentinvention will be described below with reference to the drawings. Here,cases in which voltage equalization devices of the present invention areapplied to power storage systems will be described as the embodiments ofthe present invention.

First Embodiment

FIG. 1 is a block diagram showing, in outline, the configuration of apower storage system 100 according to a first embodiment. As shown inFIG. 1, the power storage system 100 is provided with a power storagedevice 300 and a voltage equalization device 200, and the voltageequalization device 200 of this embodiment equalizes voltages bycontrolling the amounts of charging/discharging of the power storagesystem 300.

In addition, in order to enhance the reliability and safety of the powerstorage device 300, the power storage system 100 is provided withprotection circuits (not shown) that respectively perform over-chargingprotection, over-discharging protection, overcurrent protection,over-heating protection, etc. for the cells.

The power storage device 300 is provided with battery packs 20 in whichat least one secondary cell is connected, power converters 30 providedin association with the individual battery packs 20, and controllers(controlling means) 40 that control the power converters 30, and isconfigured such that the individual battery packs 20 are connected inparallel via the individual power converters 30 and so that theindividual battery packs 20 and the voltage equalization device 200 cancommunicate with each other. The battery packs 20 are formed of, forexample, lithium-ion batteries, nickel-cadmium batteries, nickelhydrogen batteries, or the like.

The battery packs 20 are basically formed of single batteries or aplurality of single batteries that are connected in series. Note that,connections of the single batteries may have not only portions connectedin series but also portions connected in parallel.

The battery packs 20 are individually connected in parallel via thepower converters 30 provided in association with the individual batterypacks 20, and battery-pack information measured by each battery pack isoutput to the voltage equalization device 200 (details thereof to bedescribed later). The battery-pack information to be measured includesinformation regarding, for example, inter-terminal voltages of theindividual battery packs, SOC (state of charge:charged power levelindicated as a ratio relative to the power capacity), which indicatesthe state of charge of the battery packs, and so on.

The controllers 40 generate voltage-control instructions on the basis ofoffset instructions calculated by the voltage equalization device 200and externally input operation instructions, and output them to theindividual power converters 30.

The power converters 30 are driven on the basis of the voltage-controlinstructions received from the controllers 40 to thereby performcharging/discharging of the battery packs connected thereto.

For example, the power converters 30 are provided with a plurality ofswitching elements (for example, thyristors, IGBTs, power MOSTFETs, orthe like), and these switching elements are driven to switch atpredetermined timing on the basis of the voltage-control instructionsreceived from the controllers 40 to thereby function as rectifiers thatrectify AC power from a power grid and supply it to the power storagedevice 300 when charging the power storage device 300 and to function asinverters that convert the power from the power storage device 300 to ACpower and output it to a load system or the like when discharging. Inaddition, by changing the duty ratio for switching on/off, the outputpower level for a certain period can be increased/decreased.Furthermore, the above-described power grid includes distributed powersources such as power plants based on wind power, fuel cells, MGT (microgas turbines), solar power or the like, in addition to generalcommercial power plants.

FIG. 2 is a block diagram showing the internal configuration of thevoltage equalization device 200. The voltage equalization device 200 isprovided with a decision portion (deciding means) 201 and avoltage-adjusting portion (voltage-adjusting means) 202.

The decision portion 201 obtains the battery-pack information regardingthe state of charging/discharging of each battery pack 20 and decides,for each battery pack, whether or not to perform voltage adjustment onthe basis of this battery-pack information. Specifically, the decisionportion 201 compares a maximum value of inter-terminal voltage of eachbattery pack with a predetermined threshold and decides that the voltageadjustment should be performed when the maximum value is at thepredetermined threshold or above.

When the decision portion 201 has decided that the voltage adjustment isto be performed, the voltage-adjusting portion 202 generates offsetinstructions for adjusting the state of charging/discharging and outputsthe offset instructions to the power controllers 40 of the powerconverters associated with the battery packs. Specifically, of thebattery information of the individual battery packs 20, thevoltage-adjusting portion 202 outputs to the controllers 40 of the powerconverters associated with the individual battery packs the offsetinstructions for performing adjustments so as to reduce the voltages ofthe battery packs having relatively large inter-terminal voltage and toincrease the voltages of the battery packs having relatively smallinter-terminal voltages.

For example, defining the maximum value of the battery-pack information(for example, inter-terminal voltage or SOC) as X_(max) and theinformation of target battery packs as X_(i), the voltage-adjustingportion 202 specifies battery packs for which X_(max)-X_(i) is equal toor greater than a predetermined threshold TH5 (hereinafter referred toas “Condition 1”) and sets current I_(c) (I_(c)>0: charging +) as theoffset instructions for the controllers 40 of the power convertersassociated with the specified battery packs. Next, the voltage-adjustingportion 202 calculates a total current SUMI_(c) (SUMI_(c)<0) of theoffset instructions set for the battery packs that satisfy Condition 1described above. For example, when the detected number of the batterypacks that satisfy Condition 1 is α, the total current is obtained asSUMI_(c)=α×I_(c)=αI_(c). Then, the voltage-adjusting portion 202determines the offset instructions for the individual battery packs thatdo not satisfy Condition 1 by dividing this SUMI_(c) by the number ofbattery packs that do not satisfy the Condition 1. For example, when thenumber of battery packs that do not satisfy Condition 1 is β, theabove-described total current SUMI_(c) is divided by β, and to determinethe offset instructions (=SUMI_(c)/β) for the battery packs that do notsatisfy Condition 1.

Accordingly, current flows from the battery packs that do not satisfyCondition 1 to the battery packs that satisfy Condition 1, and thus, thestates of charge of the individual battery packs are equalized.

Modification 1

With the power storage system in this embodiment, the offsetinstructions for the individual battery packs are determined at thevoltage-adjusting portion 202 on the basis of the maximum value of thebattery-pack information; however, instead of this method, the offsetinstructions for the individual battery packs may be determined, forexample, on the basis of a minimum value of the battery-packinformation.

Specifically, defining the minimum value of the battery-pack information(for example, inter-terminal voltage or SOC) as X_(min) and informationof the target battery packs as X_(i), the voltage-adjusting portion 202specifies battery packs for which X_(min)−X_(i) is equal to or greaterthan a predetermined threshold TH6 (hereinafter referred to as“Condition 2”) and sets current I_(c) (I_(c)>0: charging +) as theoffset instructions for the controllers 40 of the power convertersassociated with the specified battery packs. Next, the voltage-adjustingportion 202 calculates a total current SUMI_(c) (SUMI_(c)>0) of theoffset instructions set for the battery packs that satisfy Condition 2described above. For example, when the detected number of the batterypacks that satisfy Condition 2 is α, the total current is obtained asSUMI_(c)=α×I_(c)=αI_(c). Then, the voltage-adjusting portion 202determines the offset instructions for the individual battery packs thatdo not satisfy Condition 2 by dividing this SUMI_(c) by the number ofbattery packs that do not satisfy the Condition 2. For example, when thenumber of battery packs that do not satisfy Condition 2 is β, theabove-described total current SUMI_(c) is divided by β, and to determinethe offset instructions (=SUMI_(c)/β) for the battery packs that do notsatisfy Condition 2.

Accordingly, current flows from the battery packs that satisfy Condition2 to the battery packs that do not satisfy Condition 2, and thus, thestates of charge of the individual battery packs are equalized.

Modification 2

With the power storage system in this embodiment, the offsetinstructions for the individual battery packs are determined at thevoltage-adjusting portion 202 on the basis of the maximum value of thebattery-pack information; however, the offset instructions for theindividual battery packs may be determined, for example, in thefollowing manner. For example, defining an average value of thebattery-pack information (for example, inter-terminal voltage or SOC) asX_(avr) and information about the target battery packs as X_(i), theoffset instructions (charging) in accordance with the battery-packinformation of the individual battery packs are set as:I_(c)=(X_(i)−X_(avr)).

In this case, for the battery packs whose battery information is largerthan the average value, discharging is performed at current values inaccordance with the magnitude of the battery-pack information, and, forthe battery packs whose battery information is smaller than the averagevalue, charging is performed at current values in accordance with themagnitude of the battery-pack information.

Note that, with the voltage equalization device according to thisembodiment, the decision portion 201 decides whether or not to performthe equalization control by comparing the maximum value of thebattery-pack information with the predetermined threshold; however, itis not limited to this. For example, the minimum value of the individualitems of battery-pack information may be compared with the predeterminedthreshold, or the average value thereof may be compared with thepredetermined threshold. In addition, difference information, such asthe difference between a maximum value and an average value, thedifference between a minimum value and the average value, the differencebetween the maximum value and the minimum value, and so on, may becompared with the predetermined threshold for the individual items ofbattery-pack information.

More specifically, the decision portion 201 may decide whether or not toperform the voltage adjustment in accordance with whether or not themaximum value X_(max) of the inter-terminal voltage of the battery packsdescribed above is equal to or greater than a predetermined thresholdTH1 (X_(max)≧TH1); whether or not the minimum value X_(min) of theinter-terminal voltage of the battery packs is equal to or less than apredetermined threshold TH2 (X_(min)≦TH2); whether or not a differencebetween the maximum value X_(max) and the minimum value X_(min) is equalto or greater than a threshold TH3 (X_(max)−X_(min)≧TH3); or whether ornot a larger value of a value determined by subtracting the averagevalue X_(avr) of the inter-terminal voltage of the battery packs fromthe maximum value X_(max) thereof and a value determined by subtractingthe minimum value X_(min) of the inter-terminal voltage of the batterypacks from the average value X_(avr) thereof is equal to or greater thana predetermined threshold (MAX(X_(max)−X_(avr), X_(avr)−X_(min))≧TH4).

Upon determining offset instructions required for equalizing thevoltages of the individual battery packs on the basis of thebattery-pack information obtained from the individual battery packs, thevoltage-adjusting portion 202 outputs these offset instructions to thecorresponding controllers 40.

Next, the operation of the power storage system 100 according to thisembodiment will be described using FIG. 3. First, the individual batterypacks 20 calculate respective items of battery-pack information andoutput them to the voltage equalization device 200 (Step SA1 in FIG. 3).At the decision portion 201 of the voltage equalization device 200, theobtained individual items of battery-pack information are compared withthe predetermined threshold; it is decided whether or not to perform thevoltage adjustment; and the decision results are output to thevoltage-adjusting portion 202 (Step SA2).

When it is decided at the decision portion 201 that the voltageadjustment is to be performed, at the voltage-adjusting portion 202, theoffset instructions based on the battery-pack information about theindividual battery packs are individually determined for the batterypacks, and the determined offset instructions are sent to thecontrollers of the power converters associated with the respectivebattery packs (Step SA3). At the controllers 40, voltage-controlinstructions in which the obtained offset instructions andnormal-operation instructions are combined are generated, and thesevoltage-control instructions are output to the power converters 30. Atthe power converters 30, the charging/discharging of the battery packsis controlled on the basis of the voltage-control instructions obtainedfrom the controllers 40.

Note that, in the embodiment described above, processing by hardware isassumed for the voltage equalization device; however, there is no needto be limited to such a configuration. For example, a configuration inwhich processing is performed by separate software (a voltageequalization program) is also possible. In this case, the voltageequalization device is provided with a main storage device such as aCPU, RAM or the like and a computer-readable storage medium that storesa program for realizing all or part of the above-described processing.Then, the CPU reads out the program stored in the storage mediumdescribed above and executes processing and computational processing ofthe information, thereby realizing processing similar to that in theabove-described fault diagnosis device.

Here, the computer-readable storage medium refers to a magnetic disk, amagneto-optical disk, a CD-ROM, a DVD-ROM, a semiconductor memory, orthe like. In addition, this computer program may be distributed tocomputer via a communication line so that the computer that has receivedthe distributed program executes the program.

As has been described above, with the voltage equalization deviceaccording to this embodiment, the individual battery packs 20individually calculate the battery-pack information, and the decisionportion 201 constituting the voltage equalization device 200 comparesthe battery-pack information with the predetermined threshold. Then, inaccordance with the comparison results, the voltage-adjusting portion202 generates the offset instructions, and the power converters 30perform the charge/discharge control for the battery packs 20 on thebasis of the offset instructions. Accordingly, the voltage can becontrolled for each battery pack, and an advantage is afforded in thatvoltage differences in the individual battery packs are eliminated.

Second Embodiment

Next, a second embodiment of the present invention will be describedusing FIG. 4.

A voltage equalization device of this embodiment differs from the one inthe first embodiment in that bypass impedance elements 10 areindividually provided in parallel for each of the battery packs 20; theswitching elements 11 are provided in series with the bypass impedanceelements 10; and a voltage equalization device 200′ is provided insteadof the voltage equalization device 200. In the following, the voltageequalization device of this embodiment will be described with regardonly to points that are different from the first embodiment, and thedescriptions of points common therewith will be omitted.

The bypass impedance elements 10 are individually connected in parallelwith the battery packs 20 and, furthermore, the switching elements 11(details thereof to be described later) are connected in series to thebypass impedance elements 10. Here, the bypass impedance elements are,for example, resistance components.

The voltage equalization device 200′ performs on/off control of theswitching elements 11. When for example, the voltage of a certainbattery pack 20 needs to be rapidly lowed, the voltage equalizationdevice 200′ turns on the switching element 11 connected in parallel withthis battery pack 20, thereby flowing current in the bypass impedanceelement 10 connected in series with the switching element 11 that hasbeen turned on. Accordingly, discharging of the battery pack 20 can beaccelerated.

The voltage equalization device 200′ is provided with the decisionportion 201 and a voltage-adjusting portion 202′.

For example, defining the maximum value of the battery-pack informationas X_(max) and information about the target battery packs as X_(i), thevoltage-adjusting portion 202′ sends instructions for turning on theswitching elements 11 to the battery packs connected in paralleltherewith, for which X_(max)−X_(i) is equal to or greater than apredetermined threshold.

In addition, without limitation to the method described above, forexample, when X_(min)−X_(i) is equal to or greater than a predeterminedthreshold, where X_(min) is the minimum value of the battery-packinformation and X_(i) is battery-pack information about target batterypacks, the voltage-adjusting portion 202′ may turn on the relevantswitching elements 11. In addition, the switching elements 11 may beturned on when X_(i)−X_(avr) is equal to or greater than a predeterminedthreshold, where X_(avr) is the average of the battery packs.

When the decision portion 201 decides that a considerable voltageadjustment is to be performed, the voltage-adjusting portion 202′ sendsinstructions for turning on the switching elements 11 connected parallelwith the battery packs for which voltage adjustment is to be performed.Accordingly, closed circuits are formed in circuits connected inparallel with the battery packs for which voltage adjustment is to beperformed, thereby making it possible to flow current in the bypassimpedance elements 10.

As has been described above, with the power storage system according tothis embodiment bypass impedance elements are individually connected inparallel with each of the battery packs, and the switching elements areprovided in series with the bypass impedance elements. Accordingly, whenthe switching elements are turned on, the individual battery packs canquickly discharge.

In addition, because all charge/discharge control is performed via thepower converters in the first embodiment described above, even when itis desirable to accelerate charging/discharging of the battery packs, itis conceivable that the charging/discharging current will end uprestricted in accordance with the current range or output requirementinstructions of the power converters. Even in such cases, however, byproviding the bypass impedance elements in parallel with the individualbattery packs, ideal battery conditions can be rapidly attained viapower adjustment by the power converters in combination with dischargethrough the bypass impedance elements without putting a load on thepower converters.

Note that, with the power storage system in this embodiment, thevoltage-adjusting portion 202′ controls the switching elements 11;however, it is not limited to this. For example, when the voltagedifference is small and slight voltage adjustment is performed for eachbattery pack, the power converters 30 may be controlled while keepingthe switching elements 11 in the off state.

In addition, with the power storage system in this embodiment, whenconnecting the bypass impedance elements and the switching elements inseries, although the bypass impedance elements are connected at positivesides of the power converters, and the switching elements are connectedat negative sides of the power converters in FIG. 4, it is not limitedto this. For example, the bypass impedance elements may be connected atthe negative sides of the power converters, and the switching elementsmay be connected at the positive sides of the power converters.

In addition, with the voltage equalization device according to thisembodiment, although the bypass impedance elements 10 and the switchingelements 11 are individually provided in parallel for each of thebattery packs in FIG. 4, it is not limited to this. For example, theymay individually be provided for cells constituting the battery packs 20or for cell groups formed by dividing the battery packs 20 into aplurality of groups.

Third Embodiment

Next, a third embodiment of the present invention will be describedusing FIG. 5.

A voltage equalization device of this embodiment differs from the onesin the first and second embodiments in that a voltage equalizationdevice 200″ is provided instead of the voltage equalization device 200.In the voltage equalization device 200″, of the battery packs 20 inwhich a plurality of secondary cells are connected, an impedance element10 is provided in parallel between one of the battery packs 20 and theother battery packs other than the one battery pack 20.

With the impedance element 10 of the voltage equalization device 200″,when there are battery packs with higher voltages among the individualbattery packs, current flows from the battery packs with higher voltagesto the battery packs with lower voltages.

In this way, by flowing the current among the individual battery packsvia the impedance element 10, voltage differences can be eliminatedamong battery packs connected to the impedance element 10 through whichthe current has flowed.

Modification 1

With the voltage equalization device 200″ according to this embodiment,the impedance element 10 is provided between one of the battery packs 20and the other battery packs other than the one battery pack 20; however,it is not limited to this. For example, of the plurality of the batterypacks 20, adjacent battery packs may be connected in parallel with eachother via impedance elements and two battery packs located at both endsmay be connected in parallel via an impedance element.

In this way, of the plurality of the battery packs 20, adjacent batterypacks are connected in parallel with each other via the impedanceelements and the two battery packs located at both ends may be connectedin parallel via an impedance element; therefore, the battery packs canbe connected in a ring-like manner via the impedance elements.Accordingly, as compared with connecting one battery pack with the otherbattery packs via an impedance element, the number of current routescommunicating from high-voltage battery packs to low-voltage batterypacks can be increased, thereby making it possible to enhance theefficiency of the voltage equalization.

Modification 2

With the voltage equalization device 200″ according to this embodiment,the impedance element 10 is provided between one of the battery packs 20and the other battery packs other than the one battery pack 20; however,it is not limited to this. For example, of the plurality of the batterypacks 20, the battery packs that are not adjacent to each other may beconnected in parallel via impedance elements.

Because battery packs among the plurality of the battery packs that arenot adjacent to each other are connected in parallel via the impedanceelements in this way, the number of current routes communicating fromhigh-voltage battery packs to low-voltage battery packs can be furtherincreased, and thus, it becomes possible to further enhance theefficiency of the voltage equalization.

Modification 3

With the voltage equalization device 200″ according to this embodiment,the impedance elements 10 are provided between the individual batterypacks; however, it is not limited to this. For example, switchingelements 11 may be provided in series with the impedance elements 10,and switching controllers that control the switching elements 11 may beprovided.

In this way, the switching elements are turned on based on therespective items of battery-pack information obtained from theindividual battery packs, thereby making it possible to performcharging/discharging of the battery packs 20 via the impedance elements.Accordingly, it becomes possible to selectively performcharging/discharging of the battery packs 20 via the impedance elements.

REFERENCE SIGNS LIST

-   10 bypass impedance element-   11 switching element-   20 battery pack-   30 power converter-   40 controller-   100 power storage system-   200 voltage equalization device-   201 decision portion-   202 voltage-adjusting portion

The invention claimed is:
 1. A voltage equalization device for a powerstorage device which includes a plurality of battery packs and aplurality of power converters and in which the plurality of batterypacks are connected in parallel via each of the plurality of powerconverters, the plurality of power converters being respectivelyassociated with the plurality of battery packs and performing voltageadjustment for the respectively associated battery packs, the voltageequalization device comprising: deciding means for obtainingbattery-pack information which is information regarding states ofcharge/discharge of each of the plurality of battery packs and fordeciding, for each of the plurality of battery packs, whether or not thedifference between the value indicated by the obtained battery-packinformation and a previously-determined value as a reference value ofthe battery-pack information is equal to or more than a predeterminedthreshold; a plurality of bypass impedance elements that arerespectively connected in parallel with the plurality of battery packs;a plurality of switching elements that are respectively associated withthe plurality of battery packs and that are respectively connected inseries with the plurality of bypass elements; first voltage-adjustingmeans for causing each of the plurality of power converters to performvoltage adjustment; and second voltage-adjusting means for controllingeach of the plurality of switching elements, wherein the secondvoltage-adjusting means turns on one of the plurality of switchingelements associated with one of the plurality of battery packs for whichthe deciding means has decided that the difference is equal to or morethan the predetermined threshold; the first voltage-adjusting meanscauses the power converter associated with the one of the plurality ofbattery packs to perform voltage adjustment; the secondvoltage-adjusting means turns off another one of the plurality ofswitching elements associated with another one of the plurality ofbattery packs for which the deciding means has decided that thedifference is less than the predetermined threshold; and the firstvoltage-adjusting means causes the power converter associated with theanother one of the plurality of battery pack to perform voltageadjustment.
 2. A voltage equalization device for a power storage devicewhich includes a plurality of battery packs and a plurality of powerconverters and in which the plurality of battery packs are connected inparallel via each of the plurality of power converters, the plurality ofpower converters being respectively associated with the plurality ofbattery packs and performing voltage adjustment for the respectivelyassociated battery packs, the voltage equalization device comprising:deciding means for obtaining battery-pack information which isinformation regarding states of charge/discharge of each of theplurality of battery packs and for deciding, for each of the pluralityof battery packs, whether or not the difference between the valueindicated by the obtained battery-pack information and apreviously-determined value as a reference value of the battery-packinformation is equal to or more than a predetermined threshold; aplurality of bypass impedance elements that are provided for individualunit cells constituting the battery packs or for each cell group formedby dividing the battery packs into a plurality of groups, a plurality ofswitching elements that are respectively associated with the pluralityof battery packs and that are respectively connected in series with theplurality of bypass elements; first voltage-adjusting means for causingeach of the plurality of power converters to perform voltage adjustment;and second voltage-adjusting means for controlling each of the pluralityof switching elements, wherein the second voltage-adjusting means turnson one of the plurality of switching elements associated with one of theplurality of battery packs for which the deciding means has decided thatthe difference is equal to or more than the predetermined threshold; thefirst voltage-adjusting means causes the power converter associated withthe one of the plurality of battery packs to perform voltage adjustment;the second voltage-adjusting means turns off another one of theplurality of switching elements associated with another one of theplurality of battery packs for which the deciding means has decided thatthe difference is less than the predetermined threshold; and the firstvoltage-adjusting means causes the power converter associated with theanother one of the plurality of battery pack to perform voltageadjustment.
 3. The voltage equalization device according to claim 1,wherein the first voltage-adjusting means generates, for each of theplurality of power converters, offset instructions for adjusting thestates of charge/discharge on the basis of results of the decision bythe deciding means; and the plurality of power converters adjust thestates of charge/discharge on the respectively associated battery packson the basis of the offset instruction generated by the firstvoltage-adjusting means.
 4. A voltage equalization method in a voltageequalization device for a power storage device which includes aplurality of battery packs and a plurality of power converters which arerespectively associated with the plurality of battery packs and whichperform voltage adjustment for the respectively associated batterypacks, and in which the plurality of battery packs are connected inparallel via each of the plurality of power converters, the voltageequalization device including a plurality of bypass impedance elementsthat are provided for individual unit cells constituting the batterypacks or for each cell group formed by dividing the battery packs into aplurality of groups and a plurality of switching elements that arerespectively associated with the plurality of battery packs and that arerespectively connected in series with the plurality of bypass elements,the method comprising: an obtaining step of obtaining battery-packinformation which is information regarding states of charge/discharge ofeach of the plurality of battery packs; a deciding step of deciding, foreach of the plurality of battery packs, whether or not the differencebetween the value indicated by the obtained battery-pack information anda previously-determined value as a reference value of the battery-packinformation is equal to or more than a predetermined threshold; a firstvoltage-adjusting step of turning on one of the plurality of switchingelement associated with one of the plurality of battery packs for whichthe deciding step has decided that the difference is equal to or morethan the predetermined threshold and causing the power converterassociated with the one of the plurality of battery packs to performvoltage adjustment; and a second voltage-adjusting step of turning offanother one of the plurality of switching elements associated withanother one of the plurality of battery packs for which the decidingstep has decided that the difference is less than the predeterminedthreshold and causing the power converter associated with the anotherone of the plurality of battery packs to perform voltage adjustment.